动态创建case类

Question

我正在使用一个csv库，它接受一个case类并将其转换为行供我阅读。

语法与File(path).asCsvReader[caseClass]非常接近。链接到这里的库

然而，问题是我想从数据库中的表生成我的case类。我可以在我的数据库中接收表和列的类型(Int, Long, Double, String等)，但我不知道如何动态创建一个case类与该数据，因为我不知道在编译时的信息。

正因为如此，我也不能使用宏，因为我不知道宏编译时的表数据。

那么，一旦我接收到表格数据，然后将case类传递给csv库，我该如何动态地创建这个case类呢?

Answer 1

根据devim在这个答案中的提示，我添加了第二个解决方案，它可以在Scala 2.10和2.11中工作，并使用Scala工具箱。不幸的是，生成的case类位于默认包中。

生成案例类

/**
  * Generate a case class and persist to file
  * Can't add package to the external class
  * @param file External file to write to
  * @param className Class name
  * @param header case class parameters
  */
def writeCaseClass(file: File, className: String, header: String): Unit = {
  val writer: PrintWriter = new PrintWriter(file)
  writer.println("case class " + className + "(")
  writer.println(header)
  writer.println(") {}")
  writer.println("nscala.reflect.classTag[" + className + "].runtimeClass")
  writer.flush()
  writer.close()
}

使用Toolbox

编译外部类

/**
  * Helper method to search code files in directory
  * @param dir Directory to search in
  * @return
  */
def recursiveListFiles(dir: File): Array[File] = {
  val these = dir.listFiles
  these ++ these.filter(_.isDirectory).flatMap(recursiveListFiles)
}  
/**
  * Compile scala files and keep them loaded in memory
  * @param classDir Directory storing the generated scala files
  * @throws IOException if there is problem reading the source files
  * @return Map containing Class name -> Compiled Class Reference
  */
@throws[IOException]
def compileFiles(classDir: String): Map[String, Class[_]] = {
  val tb = universe.runtimeMirror(getClass.getClassLoader).mkToolBox()
  val files = recursiveListFiles(new File(classDir))
    .filter(_.getName.endsWith("scala"))
  println("Loaded files: n" + files.mkString("[", ",n", "]"))
  files.map(f => {
    val src = Source.fromFile(f).mkString
    val clazz = tb.compile(tb.parse(src))().asInstanceOf[Class[_]]
    getClassName(f.getName) -> clazz
  }).toMap
}

实例化和使用编译后的类

编译后的类可以从映射中获得并在必要时使用，例如

//Test Address class
def testAddress(map: Map[String, Class[_]]) = {
  val addressClass = map("AddressData")
  val ctor = addressClass.getDeclaredConstructors()(0)
  val instance = ctor.newInstance("123 abc str", "apt 1", "Hello world", "HW", "12345")
  //println("Instantiated class: " + instance.getClass.getName)
  println(instance.toString)
}

Answer 2

如果您使用的是Scala 2.10，您可以使用scala.tools.nsc.interpreter包中的类来完成此操作。注意，这在Scala 2.11中不再起作用。我问了一个新问题，希望我们能得到答案。

在Scala 2.10中，您可以使用解释器编译外部类文件并虚拟加载它。

步骤如下:

• 根据约定为你的类命名
• 解析您的CSV头以了解字段名称和数据类型
• 用上述信息生成一个case类，并写入磁盘
上的文件
• 加载生成的源文件并使用解释器编译它们
• 类现在可以使用了。

我已经建立了一个小的演示，应该有帮助。

/**
  * Location to store temporary scala source files with generated case classes
  */
val classLocation: String = "/tmp/dynacode"
/**
  * Package name to store the case classes
  */
val dynaPackage: String = "com.example.dynacsv"
/**
  * Construct this data based on your data model e.g. see data type for Person and Address below.
  * Notice the format of header, it can be substituted directly in a case class definition.
  */
val personCsv: String = "PersonData.csv"
val personHeader: String = "title: String, firstName: String, lastName: String, age: Int, height: Int, gender: Int"
val addressCsv: String = "AddressData.csv"
val addressHeader: String = "street1: String, street2: String, city: String, state: String, zipcode: String"
/**
  * Utility method to extract class name from CSV file
  * @param filename CSV file
  * @return Class name extracted from csv file name stripping out ".ext"
  */
def getClassName(filename: String): String = filename.split("\.")(0)
/**
  * Generate a case class and persist to file
  * @param file External file to write to
  * @param className Class name
  * @param header case class parameters
  */
def writeCaseClass(file: File, className: String, header: String): Unit = {
  val writer: PrintWriter = new PrintWriter(file)
  writer.println("package " + dynaPackage)
  writer.println("case class " + className + "(")
  writer.println(header)
  writer.println(") {}")
  writer.flush()
  writer.close()
}
/**
  * Generate case class and write to file
  * @param filename CSV File name (should be named ClassName.csv)
  * @param header Case class parameters. Format is comma separated: name: DataType
  * @throws IOException if there is problem writing the file
  */
@throws[IOException]
private def generateClass(filename: String, header: String) {
  val className: String = getClassName(filename)
  val fileDir: String = classLocation + File.separator + dynaPackage.replace('.', File.separatorChar)
  new File(fileDir).mkdirs
  val classFile: String = fileDir + File.separator + className + ".scala"
  val file: File = new File(classFile)
  writeCaseClass(file, className, header)
}
/**
  * Helper method to search code files in directory
  * @param dir Directory to search in
  * @return
  */
def recursiveListFiles(dir: File): Array[File] = {
  val these = dir.listFiles
  these ++ these.filter(_.isDirectory).flatMap(recursiveListFiles)
}
/**
  * Compile scala files and keep them loaded in memory
  * @param classDir Directory storing the generated scala files
  * @throws IOException if there is problem reading the source files
  * @return Classloader that contains the compiled external classes
  */
@throws[IOException]
def compileFiles(classDir: String): AbstractFileClassLoader = {
  val files = recursiveListFiles(new File(classDir))
                  .filter(_.getName.endsWith("scala"))
  println("Loaded files: n" + files.mkString("[", ",n", "]"))
  val settings: GenericRunnerSettings = new GenericRunnerSettings(err => println("Interpretor error: " + err))
  settings.usejavacp.value = true
  val interpreter: IMain = new IMain(settings)
  files.foreach(f => {
    interpreter.compileSources(new BatchSourceFile(AbstractFile.getFile(f)))
  })
  interpreter.getInterpreterClassLoader()
}
//Test Address class
def testAddress(classLoader: AbstractFileClassLoader) = {
  val addressClass = classLoader.findClass(dynaPackage + "." + getClassName(addressCsv))
  val ctor = addressClass.getDeclaredConstructors()(0)
  val instance = ctor.newInstance("123 abc str", "apt 1", "Hello world", "HW", "12345")
  println("Instantiated class: " + instance.getClass.getCanonicalName)
  println(instance.toString)
}
//Test person class
def testPerson(classLoader: AbstractFileClassLoader) = {
  val personClass = classLoader.findClass(dynaPackage + "." + getClassName(personCsv))
  val ctor = personClass.getDeclaredConstructors()(0)
  val instance = ctor.newInstance("Mr", "John", "Doe", 25: java.lang.Integer, 165: java.lang.Integer, 1: java.lang.Integer)
  println("Instantiated class: " + instance.getClass.getCanonicalName)
  println(instance.toString)
}
//Test generated classes
def testClasses(classLoader: AbstractFileClassLoader) = {
  testAddress(classLoader)
  testPerson(classLoader)
}
//Main method
def main(args: Array[String]) {
  try {
    generateClass(personCsv, personHeader)
    generateClass(addressCsv, addressHeader)
    val classLoader = compileFiles(classLocation)
    testClasses(classLoader)
  }
  catch {
    case e: Exception => e.printStackTrace()
  }
}

}

输出:

Loaded files: 
[/tmp/dynacode/com/example/dynacsv/AddressData.scala,
 /tmp/dynacode/com/example/dynacsv/PersonData.scala]
Instantiated class: com.example.dynacsv.AddressData
AddressData(123 abc str,apt 1,Hello world,HW,12345)
Instantiated class: com.example.dynacsv.PersonData
PersonData(Mr,John,Doe,25,165,1)

Answer 3

基本上你需要一个关于case类的运行时信息?我猜你应该用ClassTag:

import scala.reflect._
def asCsvReader[T: ClassTag]: T = {
  classTag[T].runtimeClass.getDeclaredConstructor(...).newInstance(...)
  ...
}

这将允许您在运行时实例化case类。

既然您可以计算出CSV列的类型，那么您可以将适当的类型提供给getDeclaredConstructor和newInstance方法。